Spectroscopic imaging combines digital imaging and molecular spectroscopy techniques, which can include Raman scattering, fluorescence, photoluminescence, ultraviolet, visible and infrared absorption spectroscopies. When applied to the chemical analysis of materials, spectroscopic imaging is commonly referred to as chemical imaging. Instruments for performing spectroscopic (i.e. chemical) imaging typically comprise image gathering optics, focal plane array imaging detectors and imaging spectrometers.
In general, the sample size determines the choice of image gathering optic. For example, a microscope is typically employed for the analysis of sub micron to millimeter spatial dimension samples. For larger objects, in the range of millimeter to meter dimensions, macro lens optics are appropriate. For samples located within relatively inaccessible environments, flexible fiberscopes or rigid borescopes can be employed. For very large scale objects, such as planetary objects, telescopes are appropriate image gathering optics.
For detection of images formed by the various optical systems, two-dimensional, imaging focal plane array (FPA) detectors are typically employed. The choice of FPA detector is governed by the spectroscopic technique employed to characterize the sample of interest. For example, silicon (Si) charge-coupled device (CCD) detectors, a type of FPA, are typically employed with visible wavelength fluorescence and Raman spectroscopic imaging systems, while indium gallium arsenide (InGaAs) FPA detectors are typically employed with near-infrared spectroscopic imaging systems.
A variety of imaging spectrometers have been devised for spectroscopic imaging systems. Examples include, without limitation, grating spectrometers, filter wheels, Sagnac interferometers, Michelson interferometers and tunable filters such as acousto-optic tunable filters (AOTFs) and liquid crystal tunable filters (LCTFs).
A number of imaging spectrometers, including AOTFs and LCTFs are polarization sensitive, passing one linear polarization and rejecting the orthogonal linear polarization. As a result, theoretical efficiency is 50%. Practical efficiency is reduced due to scattering losses, imperfect spectrometer action, absorption losses in polarizing optics, etc. Practical efficiency of 30% peak transmittance or less is more typical. Previous spectroscopic imaging instruments accepted the optical losses associated with polarization sensitive imaging spectrometers.
The efficiency of imaging spectrometers is also a function of the system-specific noise caused by background light. For example, the LCTF has a wavelength dependent transmission modulation which affect's the accuracy and the efficiency of measuring sharp Raman bands with weak Raman scatterers. Experiments with certain LCTF devices show complicated interactions arising in the material and structure of the imaging devices produce a spatial and spectral modulation of light coming through the imaging device. The modulation produces an apparent background signal that is not uniform and masks the real signal. Much of the background noise has comparable structural features (e.g., peak width) as the Raman signal. For material having weak Raman scattering or samples having a low concentration of the material being studied, the background noise can mask or obscure the real signal. Because the ability to detect Raman vibrational features on a uniform background is critical to identifying small signal levels, the background noise can have a detrimental affect on the spectral detection.